His book on _Earth Sculpture_ followed in 1898, and in this particular field soon came to be recognised as a standard work. It was published as one of a series, and the limitations of space probably did not allow very complete discussion of so large a subject: in fact, it is only a sketch of the relations between geology and surface features; but the subject was one for which he had a great liking, and he dwelt on it lovingly in the lectures which he delivered each winter to the University students. In this, of course, he followed the Scottish tradition, which since the days of Hutton and Playfair had assigned to this branch of geology a special importance. The writing of this little book accordingly was a real pleasure to him, and he drew nearly all the illustrations for it with his own hand, feeling that the only difficulty was to keep himself within the limits which necessity imposed. His immense knowledge of geographical literature supplied him with abundant material to illustrate the operation of natural agents in giving rise to modifications of topographical form, and the change of subject-matter from the always more or less controversial questions of the glacial history of the northern hemisphere afforded stimulus to his pen. CHAPTER XIII INTERGLACIAL CONTROVERSIES The second edition of _The Great Ice Age_ was sold out about the year 1892, and the author set himself to the task of preparing a third edition, incorporating all the most recent investigations. The task was necessarily a very severe one, as an enormous increase had taken place in the number of workers in this field, and the evidence had accumulated at a very rapid rate. Prof. Geikie always regarded this work as his principal contribution to geological literature, an opinion which most of his critics seem to have shared; and certainly the labour he spent on the third edition of his _magnum opus_ was sufficient to entitle it to a high place in the history of glacial investigation. With indomitable perseverance he undertook the work of mastering the literature, and the number of papers he read may be inferred from the fact that his collection of pleistocene and glacial pamphlets, which now form a part of the noble library of the University of Edinburgh, numbered over one hundred and sixty volumes. He aimed at making the book not only a compendium of information on the subject of which it treated, but also a critical review of the conclusions which might fairly be drawn from the evidence to hand; and in this he not unfrequently differed from the authors of the papers he cited, a proceeding which was likely to awaken feelings the reverse of grateful.